In a typical cellular system, also referred to as a wireless communications network, wireless terminals, also known as mobile stations and/or User Equipment units (UEs) communicate via Radio Access Networks (RAN) to a Core Network (CN). The wireless terminals may be mobile stations or user equipments such as mobile telephones also known as cellular telephones, and laptops with wireless capability, e.g., mobile termination, and thus may be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node, such as a base station, which in some radio access networks is also called eNodeB (eNB), NodeB, or base station. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations. There are different types of radio network nodes/base stations, such as for example macro node/base station, pico node/base station, home eNodeB or femto base station. Typically, the types of base stations are associated with different power classes, e.g. a typical maximum transmit power of macro base station (aka wide-area base station) is above 40 dBm, whilst lower-power base stations such as pico of femto typically have the output power below 30 dBm.
The interest in deploying low-power nodes, such as pico base stations, home eNodeBs (HeNB, HBS), relays, remote radio heads, etc., for enhancing macro network performance in terms of the network coverage, capacity, and service experience of individual users has been constantly increasing over the last few years. At the same time, it has been realized the need for enhanced interference management techniques to address the interference issues caused, for example, by a significant transmit power variation among different cells and cell association techniques developed earlier for more uniform networks.
In 3rd Generation Partnership Project (3GPP), heterogeneous network deployments have been defined as deployments where low-power nodes of different transmit powers are placed throughout a macro-cell layout, implying also non-uniform traffic distribution. Such deployments are, for example, effective for capacity extension in certain areas, so-called traffic hotspots, i.e., small geographical areas with higher user density and/or higher traffic intensity where installation of pico nodes can be considered to enhance performance. Heterogeneous deployments may also be viewed as a way of densifying networks to adopt for the traffic needs and the environment. However, heterogeneous deployments bring also challenges for which the network has to be prepared for to ensure efficient network operation and superior user experience.
In heterogeneous networks, a mixture of cells of differently sized and overlapping coverage areas are deployed. A cell is a geographical area where radio coverage is provided by a base station. More than one cell can be associated with one base station. One example of such cell deployment may be a network comprising pico cells deployed within the coverage area of a macro cell. The pico cells and macro cell may each comprise a base station. A base station may be e.g. a pico base station, a macro base station, Home Base Station (HBS), radio base station, evolved node B (eNB), base station, relay, remote radio heads etc.
A base station comprises at least one antenna port, e.g. antenna port 0. Each antenna port is configured to transmit and receive signals from the base station to e.g. one or more user equipment.
Other examples of low-power nodes in heterogeneous networks are home base stations (HBS) and relays. As discussed below, the large difference in transmitted output power, e.g., 46 dBm in macro cells and less than 30 dBm in pico cells, results in an interference situation different from that seen in networks where all base stations have the same output power.
A Long Term Evolution (LTE) system uses Orthogonal Frequency Division Multiplex (OFDM) as an OFDM Access technique (OFDMA) in the downlink from system nodes to user equipments (UEs) 505, and Discrete Fourier Transform (DFT)-spread OFDM in the uplink from a user equipment 505 to an eNB. LTE channels are described in 3GPP Technical Specification (TS) 36.211 V9.1.0, Physical Channels and Modulation is described in Release 9 of LTE, among other specifications. An LTE system is used as an example in this document. However other network standards, such as GPRS, WiMAX, UMTS etc. are also applicable.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds (ms) duration, each radio frame 101 comprises ten equally-sized subframes 103 of 1 ms duration as illustrated in FIG. 1. A subframe 103 is divided into two slots, each of 0.5 ms duration. Time domain is a term used to describe the analysis of physical signals, with respect to time.
The resource allocation in LTE is described in terms of resource blocks, where a resource block corresponds to one slot in the time domain and 12 contiguous 15 kHz subcarriers in the frequency domain. Two consecutive, i.e. in time, resource blocks represent a resource block pair and correspond to the time interval upon which scheduling operates.
Reference Signals
The use of multiple antennas plays an important role in modern wireless communication systems, such as 3rd Generation (3G) LTE systems, to achieve improved system performance, including capacity and coverage, and service provisioning. Acquisition of channel state information (CSI) at the transmitter or the receiver is important to proper implementation of multi-antenna techniques. In general, channel characteristics, such as the impulse response, are estimated by sending and receiving one or more predefined training sequences, which can also be called Reference Signals (RS). To estimate the channel characteristics of a DL for example, a base station 503 transmits reference signals to user equipments 505, which use the received versions of the known reference signals to estimate the DL channel, e.g. to provide an estimated channel matrix. The user equipments can then use the estimated channel matrix for coherent demodulation of the received DL signal, and obtain potential beam-forming gain, spatial diversity gain, and spatial multiplexing gain available with multiple antennas. In addition, the reference signals may be used to do channel quality measurement to support link adaptation.
Beam-forming is a signal processing technique used to control the directionality of the reception or transmission of a signal. Spatial diversity refers to using two or more antennas to improve the quality and reliability of a wireless link. Using multiple antennas offers a receiver several observations of the same signal. Spatial Multiplexing Gain is obtained when a system is transmitting different streams of data from the same radio resource in separate spatial dimensions. Data is hence sent and received over multiple channels—linked to different pilot frequencies, over multiple antennas.
Transmissions in a network using OFDM may be seen as a grid in time and frequency. The scheduler in the base station may allocate a specific number of subcarriers during a specific time to one user equipment. To simplify the system, too small units cannot be allocated to one user equipment, and the smallest unit within OFDM is referred to as a resource element, and that is one OFDM symbol transferred on one carrier. In the case of OFDM transmission a straightforward design of a reference signal is to transmit known reference symbols in an OFDM frequency-vs.-time grid. Cell-specific Reference Signals (CRS) and symbols are described in Clauses 6.10 and 6.11 of 3GPP TS 36.211. Up to four cell-specific reference signals corresponding to up to four transmit antennas of an eNodeB are specified. There is one reference signal transmitted per downlink antenna port. Among the aforementioned reference signals, only CRS have to be transmitted in every downlink subframe, and the other RS are transmitted at specific occasions configured by the network.
LTE uses four types of downlink reference signals (RS):                Cell-specific reference signals, associated with non Multimedia Broadcast/Multicast Service Single Frequency Network (MBSFN) transmission.        MBSFN reference signals, associated with MBSFN transmission.        UE-specific reference signals.        Positioning reference signals.        
The reference signals are referred to as RS in some of the figures.
Cell-Specific Reference Signals
CRS are transmitted in the downlink from an eNB, or base station, to a user equipment, or terminal, every subframe and over the entire system bandwidth, from antenna ports 0, 1, 2 or 3. In non-MBSFN subframes, cell-specific reference signals (CRS) are transmitted on the resource elements shown in FIGS. 2a-c, for the case of a normal cyclic prefix. In telecommunications, the term cyclic prefix refers to the prefixing of a symbol with a repetition of the end. In subframes used for MBSFN transmissions, only the first two symbols may be used for CRS. FIGS. 2a-c illustrates a resource grid of sub-carriers and available OFDM symbols for antenna ports. Each element in the resource grid is called a resource element. Each resource element is used to transmit a reference signal on one antenna port.
FIG. 2a illustrates CRS transmission from one antenna port, FIG. 2b illustrates CRS transmission from two antenna ports and FIG. 2c illustrates CRS transmission from four antenna ports. The x-axis of the FIGS. 2a-c are time slots In FIGS. 2a-c, the notation Rp is used to denote a resource element used for reference signal transmission on antenna port p. The hatched resource elements without any text indicate resource elements which are not used for transmission on the antenna port of interest. The hatched resource elements with text, Rp, indicate reference symbols transmitted on the antenna port of interest. For example, in FIG. 2b, reference signals R1 is located in the first OFDM symbol (1st RS) and 3rd to the last OFDM symbol (2nd RS).
Different cells can use 6 different shifts in frequency, and 504 different signals exist. The frequency shifts are cell-specific and depend on Physical layer Cell Identity (PCI). The relation between the PCI and the CRS frequency shift is given by Vshift=NIDcell mod 6, i.e., formally up to six-reuse may be configured for CRS. In practice, however, the effective reuse depends on the number of transmitting antenna ports. As may be seen from FIG. 3, CRS have a reuse-six pattern for CRS transmitted from 1 antenna port and reuse-three for 2 to 4 antenna ports.
CRS measurements are used at least for control channel demodulation, mobility measurements, e.g. Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ), and channel estimation. When measuring RSRP and RSRQ, the user equipment 505 measures over a measured bandwidth, which can be smaller than the system bandwidth, which may be decided by the user equipment. The number of antenna ports used for CRS transmissions is configured by the network and is communicated to user equipments as a part of the system information broadcasted in the cell, but the user equipments expect CRS to be transmitted at least from one antenna port, e.g. port 0.
One advantage of transmitting the CRS from multiple antenna ports is a higher processing gain and thus more accurate measurement and potentially a shorter measurement time. The measurement refers to measurements performed on CRS, e.g. Radio Resource Management (RRM) measurements, positioning measurements etc. Furthermore, CRS from multiple antenna ports is needed for channel estimation for multi-antenna transmissions where different data streams are transmitted on different antenna ports. In the latter case, the CRS transmitted on each multiple-antenna port needs to be different, i.e. antenna port specific CRS.
Downlink Control Channels in LTE
Transmissions in LTE are dynamically scheduled in each subframe where the base station transmits assignments and/or grants to certain user equipments via a Physical Downlink Control Channel (PDCCH), which is transmitted in the first OFDM symbol(s) in each subframe and spans over the whole system bandwidth. A user equipment that has decoded downlink control information, carried by a PDCCH, knows which resource elements in the subframe that contain data aimed for the user equipment.
Demodulation of received data requires estimation of the radio channel, which is done by using transmitted reference symbols, i.e., symbols known by the receiver. For example, in LTE, cell-specific reference symbols are transmitted in all downlink subframes and, in addition to assist downlink channel estimation, they are also used for mobility measurements performed by the user equipments. LTE supports also UE-specific reference symbols aimed only for assisting channel estimation.
FIG. 3 illustrates an exemplary mapping of physical control and data channels and cell specific reference signals on resource elements in a downlink subframe. In this example, the PDCCHs occupy the first 301 out of three possible OFDM symbols, and so in this particular case, the mapping of data may start already at the second 303 OFDM symbol.
The length of the control region, which may vary subframe to subframe, is conveyed in the Physical Control Format Indicator Channel (PCFICH), which is transmitted within the control region, at locations known by user equipments. After a user equipment has decoded the PCFICH, it knows the size of the control region and in which OFDM symbol the data transmission starts. Also transmitted in the control region is the Physical Hybrid-Admission Request (HARQ) Indicator Channel, which carries Acknowledgement/Non Acknowledgement (ACK/NACK) responses to a user equipment to inform if the uplink data transmission in a previous subframe was successfully decoded by the base station or not.
Interference Management for RS
To ensure reliable and high-bitrate transmissions, maintaining a good signal quality is required in wireless networks. The signal quality is determined by the received signal strength and its relation to the total interference and noise received by the receiver. A good network plan, which among other things includes cell planning, is a prerequisite for successful network operation, but it is static. For more efficient radio resource utilization, it has to be complemented at least by semi-static and dynamic radio resource management mechanisms, which are also intended to facilitate interference management, and deploying more advanced antenna technologies and algorithms.
One way to handle interference is, for example, to adopt more advanced transceiver technologies, e.g., by implementation of interference cancellation mechanisms in terminals. Another way, which may be complementary to the former, is to design efficient interference coordination algorithms and transmission schemes in the network.
Inter-cell interference coordination (ICIC) methods for coordinating data transmissions between cells have been specified in LTE Release 8, where the exchange of ICIC information between cells in LTE is carried out via an X2 interface according to a specified X2-AP protocol. The X2 interface is the interface between to neighboring base stations. Based on this information, the network can dynamically coordinate data transmissions in different cells in the time-frequency domain and also by power control so that the negative impact of inter-cell interference is minimized.
In the current 3GPP specifications, ICIC possibilities for control channels are more limited. One approach of handling the interference on control channels is illustrated in FIG. 4, where an interfering cell, e.g., a macro cell, does not transmit PDCCHs, and thus no data, in some subframes 401, although other control channels may still be transmitted. The other cells, e.g., pico cells, are aware of the locations of these low interference subframes 401 in time and can prioritize scheduling in those subframes the user equipments which otherwise potentially may strongly suffer from the interference caused by the interfering cell. From the legacy terminal point of view, CRS still need to be transmitted in all subframes, so there will be inter-cell interference from CRS. In FIG. 4, a thin box illustrates the control region, and the broad box illustrates the data region. A sub frame 401 comprises one control region and one data region.
Given more flexibility, many techniques exist for managing interference to and from data channels, e.g., various time-division and frequency-division multiplexing schemes. The possibilities to efficiently mitigate inter-cell interference to and from control channels are limited with the current standard. Some examples are interleaving, time shifting, and blanking. Even less flexibility exists for dealing with interference to and from physical signals which typically have a pre-defined static resource allocation in the time-frequency space. An example of a physical signal is a reference signal.
Some techniques for mitigating inter-cell interference known from the prior-art are:                Signal cancellation, by which the channel is measured and used to restore the signal from, a limited number of, the strongest interferers. Impact on the receiver implementation and its complexity; in practice channel estimation puts a limit on how much of the signal energy that can be subtracted.        Symbol-level time shifting. No impact on the standard, but not relevant for Time Division Duplex (TDD) networks and networks providing the MBMS service.        Complete signal muting in a subframe, e.g. not transmitting CRS in some subframes for energy efficiency reasons proposed earlier in 3GPP. Non-backward compatible to Rel.8/9 user equipments which expect CRS to be transmitted at least on antenna port 0.        
Given the very limited set of possibilities listed above, there is a strong need for simple but efficient new techniques to resolve the CRS interference issue.
Indication of the Number of Antenna Ports
There exist techniques to allow a terminal to blindly detect the number of antenna ports, but such techniques increase the terminal complexity and since they are in general not required by the standard they may be not implemented in the terminals.
The number of antenna ports may be signaled by the network to the user equipment as a part of the system information, e.g., as a part of the radio resource configuration information, e.g., in the AntennaInfoDedicated or AntennaInfoCommon information elements, that is common for all user equipments and is optionally comprised in the System Information Block Type 2 (SIB2). Transmission of SIB2 is dynamically scheduled by the network and the scheduling information is transmitted to the user equipment as a part of System Information Block Type 1 (SIB1), which is transmitted with a fixed periodicity of 80 ms in a Radio Resource Control (RRC) message via the broadcast channel and repeated within 80 ms. There is a possibility to transmit the most essential system information, e.g. system bandwidth, PHICH configuration or system frame number, more frequently, for which Master Information Block (MIB) is specified which is transmitted with a fixed periodicity of 40 ms over the broadcast channel and repeats within 40 ms, but MIB does not contain the information on antenna ports.
The presence of antenna port 1 can also be indicated by the PresenceAntennaPort1 information element which is a part of an Evolved-Universal Mobile Telecommunications System Terrestrial Radio Access Network (E-UTRAN) measurement object transmitted in an RRCConnectionReconfiguration message. When PresenceAntennaPort1 is set to TRUE, the user equipment may assume that at least two cell-specific antenna ports are used in all neighboring cells.
Since they are always transmitted, CRS are a permanent source of interference to neighbor cells. Furthermore, when more than one antenna port is used for CRS in a cell, the CRS may be transmitted at a power level higher than the reference power level utilizing the free power from the unused CRS resource elements to be transmitted from another antenna port in the same symbol. The data can be transmitted in other symbols than CRS symbols; the control channels have less flexibility and thus the probability of colliding with other-cell CRS is higher. For CRS measurements, the situation is the worst in synchronized networks, where the same symbols, according to the CRS transmission pattern such as exemplified in FIG. 2, are used for CRS transmissions in all cells and these symbols with always-transmitted CRS always collide. In asynchronous network, in general the interference on CRS is more randomized; however, it may also happen that a CRS symbol collides with a symbol where a synchronization signal, e.g., Primary Synchronization Signal (PSS) or Secondary Synchronization Signal (SSS), or a broadcast signal is transmitted, which may degrade the measurement quality of those signals compared to if they were colliding with data symbols in a low loaded network.
Furthermore, although a cyclic prefix is used in LTE in order to make transmissions in neighbor symbols orthogonal, it may be so that the orthogonality is not maintained between the symbols even with carefully designed patterns orthogonal among cells when the delay spread exceed the cyclic prefix, which may happen in large cells or in cells in challenging urban environments. There exist techniques for inter-symbol interference cancellation, but the advanced techniques may significantly increase the user equipment complexity. This means that it is preferable to reduce the number of REs permanently allocated for transmissions, especially when such REs are the sources of high interference.
The interference generated by CRS becomes particularly crucial in heterogeneous network deployments where the transmit power may significantly vary by cell, e.g., a macro cell can be transmitting at 46 dBm and a pico cell can be transmitting at 24 dBm, further increasing the gap between the received interference and the received measured signal power. Thus the necessity of dealing with interference from macro-cell CRS when measuring a signal from a lower-power node has been indicated by many companies in 3GPP.
Since CRS are transmitted across a subframe, they interferer to the control channels, data channels and physical signals, e.g., CRS, as described above. The impact may be of a different significance in each case, but in general managing the CRS interference is important for improving the overall system performance.
The existing signaling is not dynamic and flexible enough to allow for dynamic switching of antenna ports when, for example, low interference subframes or almost blank subframes are configured in the network.